Method of granulating powder

ABSTRACT

THIS INVENTION RELATES TO A METHOD OF GRANULATING A FINE PRODUCT. A FINE POWDER IS DIFFICULT TO HANDLE WHEN IT IS MOLDED BY COMPRESSION. THEREFORE, THIS INVENTION PROVIDES A METHOD IN WHICH POWDERY FORMING AGENT, THAT IS, A SUBSTANCE WHICH VAPORIZES UPON HEATING, IS PREVIOUSLY MIXED WITH THE POWDER OF A GRAIN SIZE SMALLER THAN THE GRAIN SIZE DESIRED, THEN THIS MIXTURE IS COMPRESSED INTO SHAPED PIECES, THESE SHAPED PIECES THUS OBTAINED ARE HEATED TO GET THE FOAMING AGENT TO VAPORIZE, THE EXPANSION FORCE OF THIS VAPORIZATION PULVERIZES THE COMPRESSED PIECES INTO RAINS OF THE DESIRED GRAIN SIZE. THE INVENTION ALSO TEACHES A METHOD WHICH MAKES IT POSSIBLE TO FREELY CONTROL THE GRAIN SIZE DISTRIBUTION OF THE GRAINS TO BE TRAINED BY ALTERING THE GRAIN SIZE OF THE FOAMING AGENT, THE RATIO OF THE FOAMING AGENT ADDED AND THE COMPRESSION FRCE.

United States Patent US. Cl. 264-5 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a method of granulating a fine powder. A fine powder is diflicult to handle when it is molded by compression. Therefore, this invention provides a method in which a powdery foaming agent, that is, a substance which vaporizes upon heating, is previously mixed with the powder of a grain size smaller than the grain size desired, then this mixture is compressed into shaped pieces; these shaped pieces thus obtained are heated to get the foaming agent to vaporize, the expansion force of this vaporization pulverizes the compressed pieces into grains of the desired grain size. The invention also teaches a method which makes it possible to freely control the grain size distribution of the grains to be obtained by altering the grain size of the foaming agent, the ratio of the foaming agent added and the compression force.

CROSS REFERENCE TO RELATED APPLICATIONS This application is a continuation of patent application Ser. No. 876,035 filed Nov. 12, 1969 now abandoned.

This invention relates to a method of granulating powder, or more particularly, to a method of manufacturing grains by compressing a mixture of powder and a foaming agent into shaped pieces and heating these shaped pieces to pulverize them into grains.

When compressing a fine powder of metals, alloys, ceramics, etc. to mold and sinter it into a desired solid, its handling in the compression process is accompanied by difficulty because of its poor fluidity and a marked change in its apparent specific gravity, if the powder is in the condition of a fine powder. It is well known that this difficulty can be removed if the raw material fine powder is converted into coarse grains before the process, and this is referred to as granulation.

The method of granulation heretofore employed was either to dissolve a binding agent in a suitable solvent, mix the solution with the powder to be granulated in order to make it viscous, and then granulate it by tumbling, extrusion, etc., or to compress the powder into bound pieces, pulverize them mechanically by a pulverizer and granulate the powder by sieving the grains thus obtained.

When the method using a binding agent is employed, a binding agent which will become an impurity and the grains obtained cannot be used. Besides, when the compressed pieces are pulverized, a part of the pulverizer finds its way into them as an impurity through wear. Furthermore, when the method using a pulverizer is uSed, it is often the case that the sizes of the grains are not uniform and the product obtained is of a widely distributed grain size. Thus, the pulverizing method calls for skill, and it is difiicult to obtain a product of the desired grain size.

This invention has an object to provide a method of granulating powder which is free from the afore-mentioned drawbacks.

When making a sintered body, if fine powder is used as the raw material for the sintered body, the quantity compressed and molded before sintering varies every time a body is molded, and this great variation gives rise to fluctuation in the quality of the product obtained. Also, there is difiiculty in that since it is hard to expel the air among the powder particles out of the compressing vessel, compression has to be done twice. To avoid these difiiculties, it is common sense that grains having a larger grain size than powder particles should be used. The grains obtained by the method of this invention as a granular material for making such a sintered body are free from impurities. Such grains can be manufactured by the method of this invention more easily than by the granulating methods already known to the public.

In order to attain the object, according to this invention, a foaming agent or a substance which readily becomes a gas upon heating (hereinafter it will be referred to in this application as a foaming agent) is first added to the fine powder of the raw material (a metal, alloy, ceramics, etc.) and their mixture is compressed and molded. If such compressed and molded pieces are simply heated in a furnace, they spontaneously become pulverized by the evaporation of the foaming agent, and grains of a suitable size can be obtained.

As a foaming agent to be used in this invention, any substance which becomes a gas upon heating will do, provided that it meets the condition that it does not chemically react with the powder to be granulated. As a foaming agent, any of the already-known foaming agents such inorganic foaming agents as sodium hydrogen carbonate and ammonium carbonate, such organic foaming agents as 1,17-azobisforrnamide, 4,4'-oxybisbenzene, sulfonylhydazide, dinitrosopentamethylene-tetramine and otpolyoxymethylene, and organic solvents, naphthalene, etc., which readily becomes gasified or volatized upon heating may be used. Where a problem will be encountered because a particular element will be transferred into the product obtained, it is permissible if a foaming agent which does not contain that particular element is employed.

It is not particularly necessary to restrict the atmospheres for the heating for foaming. However, where the powder to be granulated may be oxidized or may undergo some other unfavorable change at the foaming temperature, it is advisable to carry out the heating in an atmosphere of an inert gas or in a vacuum.

The method of this invention makes it possible to control the grain size distribution of the granulated powder over a wide range by specially using a foaming agent in a solid state and regulating its grain size, the quantity of the foaming agent added and the compressing force on the mixture of the powder and foaming agent.

The inventor has discovered that in this case the greater the diameter of the foaming agent, generally the larger the grains of the granulated powder are, and that the grain size of the grains obtained is in inverse proportion to the ratio of the foaming agent added.

The hardness (strength) and density of the granulated grains can be varied by changing the density of the compressed pieces by altering the compressing force.

In carrying this invention into practice, if the powder is such that it is affected detrimentally in some way or other by temperatures above some degree, it is advisable to add cadmium stearate, lead stearate or zinc stearate as a foaming auxiliary for the purpose of lowering the foaming temperature of the foaming agent. Needless to say, a foaming agent having a foaming gas efliciency of 50 ml./ g. or more, for instance, is generally found more effective, and it is preferable to use it in a quantity of about 1-20 weight percent.

It is not especially necessary to limit the size of the compressed and molded pieces. From the viewpoint of production efficiency, the larger they are, the better. For the purpose of making the grain size of the granulated powder uniform, it is preferable to make them smaller. This is so because the larger the compression-molded piece is, the less uniform is the density of the interior of pressed piece=40% T.D.) by means of a press. Then it was heated to 250 C. in nitrogen gas to decompose the foaming agent. In this case, the under-mentioned five ranges of grain size of the foaming agent were used for the experiment, as a result of which granulated U0 the molded piece on other hand, if the molded Piece powder having the grain size distributions shown in Table is large, the press has to possess a large capacity com- 2 wa obtained, mensurate therewith. In the examples of embodiment de- TABLE 2 scribed hereinafter, the molded piece is given a size of 50 mm. diameter x 70 mm. high. The compression force Grain Size distribution (wt'pment) for such molded pieces is generally 2040 tons. Instance (i) (ii) (iii) (iv) (v) The method of this invention has such advantages in that the grain size distribution and hardness of the grains il i): ZZtfif obtained can easily be controlled, that no binding agent is 9, gig gig used so that there will be no such agent remaining in the 6.8 22.7 7.6 0.8 2.7 granulated powder as an impurity, and that there is less 1;; g'g 13 )3 13 likelihood of impurities finding their way into the prod- 3316 1110 8.4 3.0 1.0 uct than is true with mechanical pulverization.

Now I will describe examples in which this invention EXAMPLE 3 was actually embodied, from which the advantages and Using the foaming agent of the grain size f 10 characteristics of this invention will be made clearer. 150 of the Instance (v) of Example 2, the ratio of i EXAMPLE 1 addition to the raw material U0 powder (20p or less) T th d f U0 44 1 was varied to the under-mentioned three values. Thus, 0 e POW er 0 2 (gram Slze or 655) was granulated U0 powder having grain size distributions added 5 wt. percent or 7 wt. percent of a-polyoxymethyl- Shown in Table 3 was Obtained haYmg a gram Slze of H or less. or of loo/k390i" (a) 5 wt. percent foaming agent added to U0 powder. This mixture was compressed into a piece mm. diam- (b) 7 Wt Percent foaming agent added to U02 Powder. eter X 70 mm. by a pressure of 20 tons, and this com- (0) 10 Wt percent foaming agent added to U02 pressed P1666. was heated to 259 C. for foaming. The powder compressed p1ece then collapsed into grains again, though 30 (d) 20 wt Percent foaming agent added to U02 the grains obtained were of a larger grain size. powder The grain size distribution (wt. percent) of the larger grains thus obtained is shown in Table 1. TABLE 3 Grain sizes are classified as shown below according to Grain size distribution (wt. percent) the grain size of the foaming agent, also in the examples Instance W described later.

(i) Instances where a foaming agent of a grain size gaiifijggg of 44,0. or less was used. 840-300, 65.8 48.0 35.0 13.0 (ii) Instances where a foaming agent of a grain size 40 31% 21% 3113 of 44,0-150 was used. 2.0 5.3 7.1 11.5 (iii) Instances where a foaming agent of a grain size 118 31% &2 31g of 100;r300 was used.

(iv) Instances where a foaming agent of a grain size EXAMPLE 4 of 300# 1000/ was used 7 wt percent of a foaming agent of a grain Size of of i z i foaming agent of a gram Slze 1,000,0t-150 (Instance (b)) was added to U0 powder ,0. ,u was use (grain size 20 or less). This mixture was compressed TABLE 1 into two kinds of pieces of different densities, and then rain size distribution (wt. percent) granulated by heating to 250 C. in a nitrogen atmosm phere. As a result, granulated U0 powder of the grain foaming agent, foaming agent, size distrlbutions given in Table 4 was obtained. Grain size (M) of Instance Instance In this case, the higher density of the compressed piece granulated U02 (i) (iii) (1) (iii) brought about a greater strength and higher density of 0 85 0 the granulated grains. The results in this connection are 10 20 30 55 also shown in Table 4. As regards the strength, the 30 5 granulated grains were placed in a vessel and subjected to vibration for a fixed length of time, and the strength EXAMPLE 2 was measured by observing the reduction of rough grains Dinitrosopentamethylene-tetramine was used as a (MO -300 into fine grains (ratio of decrease of foaming agent. 5 wt. percent of this was mixed with U0 60 rough grains). As to the density, the apparent densities powder (grain size 20,11. or less) and this mixture was of the products sieved to a certain grain size were compressed into a compressed piece (density of the compared.

TABLE 4 Density of compressed Density of compressed 34% T.D. piece 40% T.D. piece Grain size Grain size Grain size distribudistribuof granulated tion (wt. Strength, Density, tlon (wt. Strength, Density, U02 powder percent) percent gJcm. percent) percent gJcm.

EXAMPLE 5 To WC powder (grain size at 44,41 or less) was added wt. percent of ammonium carbonate of a grain size of 840 1 or less. After this, the mixture was compressed into a compressed piece and pulverized into rough grains by heating to 200 C. in nitrogen gas. The grain size distribution of the product obtained was such that 83 wt. percent had grain sizes within the range of 840,u150,u..

EXAMPLE 6 To WC powder (grain size of 40,11. or less) were added 8 Wt. percent of azodicarbonamide and also 1 wt. percent of zinc stearate as an auxiliary. The mixture Was heated to 180 C. in nitrogen gas. (Although the decomposition temperature of azodicarbonamide is 200 C., its decomposition almost completes at about 150 C. when an auxiliary has been added). The grain size distribution of the WC powder obtained in this case was as shown in Table 5. Also shown in the table is the grain size distribution of the product obtained without the addition of the auxiliary. The product for which the auxiliary was used shows a somewhat larger ratio of rough grain. Probably this is because the auxiliary slows down the decomposition speed and weakens the breaking force of the gas.

If an auxiliary is used, some amount of an organic substance is left in the finished granulated powder. (In the case of the example, approximately 0.1%). For some applications, therefore, the use of an auxiliary is not desirable.

The rough grains obtained in the afore-mentioned Examples 1-6 were fed to an ordinary automatic press and sintered, at l500-1700 C. for U0 and 1400-1600 C. for WC powder. Sintered pieces of a homogeneous constitution with little variation in weight were obtained.

The fluidity of rough grains was better than that of the raw material powder directly fed into the press. Very little variation in weight took place when the grains were automatically dropped to fill the press mold.

When a granulated powder obtained by the method of this invention was heated to 1500"--1700 C. in an H atmosphere, grains of 90-97% T.D. (theoretical density) were obtained. In this way, a granulated product of a still higher density can be obtained by sintering. This is because the density of granulated powder before sintering can be made high by this method. This is a remarkable feature of this method of granulation.

If these high density sintered grains of different grain size distributions are suitably combined and filled into a pipe by the vibration filling technique or the like to a filling density of 90% or so, a powder-type fuel rod of 81-87% T.D. can be made.

Besides U0 ceramics nuclear fuels can also be granulated by the method of this invention. Uranium carbide and uranium nitride are readily oxidized in the air. The method of this invention can, therefore, be applied to them satisfactorily, if a foaming agent which does not contain 0 in the decomposition gas is used, and an inert gas is used for the heating atmosphere.

Referring to the afore-mentioned examples of embodiment, it is advisable to carry this invention into practice with the under-mentioned conditions taken into consideration as regards the grain size of the foaming agent,

the ratio of the foaming agent added and the compression force for molding.

The details concerning this invention which have been described may be summarized as follows:

(a) Grain size of the foaming agent Since the grain size of the foaming agent determines the grain size of the granule obtained, the limits to the grain size of the foaming agent are to be determined reversely by the desired grain size of the granulated product. As the grain size of the granulated powder for making sintered pieces, a size of 1 000/1.-44;.t as shown herein is preferable. This is because these grain sizes make the fluidity of the powder good and also because these grain sizes are not too large, but keep the apparent density of the powder considerably large. In consequence, the grain sizes of the foaming agents shown in the examples are used in order to obtain grains of a grain size distribution meeting the requirements.

(b) Ratio of addition The smaller the ratio of addition, the more economical it is. However, if the quantity added is too small, the expanding force for breaking the compressed piece will not be obtained, so that granulation will not take place. This lower limit of the quantity added varies according to the kind of foaming agent used, since it varies according to the volume of gas produced by 1 g. of the foaming agent. For example, the ratio of the quantities of azodicarbonamide (decomposition gas volume 270 ml./ g.) and sulfonhydrazide (decomposition gas volume 110 mL/g.) used to obtain about the same grain size distribution is approximately 1:3.

(c) Compression pressure If the pressure is low and the density of the compressed piece is too low, there will be a large proportion of fine powder after pulverization. In the case of U0 800 kg./ cm. or more is preferable. The upper limit of this pressure is determined by the following consideration. This granulated powder is made into a shaped piece and then sintered. If the density of granulated powder in this sintered piece is too high, the shaped piece may have cracks in it and have a poor appearance, and grain borders in the sintered piece may become distinct and the surface may lose its smoothness. In consequence, it is preferable that the molding density at the time of granulation does not exceed of the molding density for sintering.

I claim:

1. The method of granulating compacted fine powder materials into a selected coarser grain size comprising the steps of mixing a volatile foaming agent with the fine powder materials, compressing the mixture to form a molded piece, heating the molded piece to volatize the foaming agent from within the molded piece thereby causing the latter to fissure and break up into coarse grains.

2. The method of granulating of claim 1 characterized by the step of selectively controlling the coarse grain size by varying the ratio of the foaming agent mixed with the fine powder materials.

3. The method of granulating of claim 1 characterized by the step of selectively controlling the coarse grain size by varying the grain size of the foaming agent mixed with the fine powder materials.

4. The method of granulating of claim 1 characterized by the step of selectively controlling the coarse grain size by varying the amount of the compression used in compressing the mixture to form a molded piece.

5. The method of granulating of claim 1 characterized by the step of adding a foaming auxiliary agent to the mixture to lower the volatizing temperature of the foaming agent.

6. The method of granulating of claim 1 characterized by the step of providing an inert atmosphere during the step of heating to volatize the foaming agent.

References Cited UNITED STATES PATENTS St. Pierre 2640.5 Sermon 264-05 Akimoto et a1. 23-313 Anselin et a1. 264-05 3,447,962 6/1969 Megowen 23313 3,536,475 10/ 1970' Trub 23-313 3,536,793 10/1970 Norman et a1. 264-0.5

CARL D. QUARFORTH, Primary Examiner R. GAITHER, Assistant Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 307391048 Dated June 12, 1973 l tb Kazutoshi Morita It is certified that error appears in the above-identified patent and that said Letters .Patent are hereby corrected as shown below:

Col. 1, after line 7, insert the following:

"Claims priority, application Japan Serial No. 82,523/1968 filed November 13, 1968."

Signed and sealed this 16th day 'of Jul 1974.

(SEAL) Attest:

McCOY M. GIBSON, JR. I C. MARSHALL DANN Attesting**Officer Commissioner of Patents FORM PO-IO O (IO-69) uscoMM-Dc 60376-P69 9 U.S. GOVERNMENT PRINTING OFFICE: I969 0366-334 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 739 r 048 Dated June 12 1973 I t Kazutoshi Morita It is certified that error appears in the above-identified patent and that said Letters .Patent are hereby corrected as shown below:

Col. 1, after line 7, insert the following:

"Claims priority, application Japan Serial No. 82,523/1968 filed November 13, 1968." I

Signed and sealed this 16th day oi July 1974.

(SEAL) Attest:

MCCOY M. GIBSON, JR; c. MARSHALL DANN A Attesting"0fficer Commissioner of Patents FORM PO-1OSO (10-69) A USCOMWDC 8037mm 9 [L5, GOVERNMENT PRINTING OFFICE: I969 O366-334 

